Do the A Levels. Do NOT turn your back on education at 17 years of age with the current economic mess. Unless daddy has 60 odd Grand in the bank he can give you for flying, do your A Levels and go to University. I regrettably dropped out of school after my GCSEs.
After dropping out I got into a trade for a few years before deciding to do my Helicopter PPL. I went to Florida in 2008 with a lovely loan for 6 weeks to accomplish exactly that. After returning back to the UK with a plan to hour build towards my CPL while working, I was told by my employer there wouldn't be any work for about 2 months or more. Work did crop up but only here and there throughout the year. Due to this I didn't get to fly very much because I had to pay the dam loan plus run my car. Now I am 23 and back at college doing an Electrical Engineering Diploma before progressing onto University all going well.
Anyway as I said get your heads into them books and do the A Levels. This is one piece of paper you need for gaining entrance to Military flight training. I've always dreamed of flying a Chinook for the RAF but it won't happen now because I was silly enough to leave school early and not do better.
You can do Aerospace Engineering Degrees at University now that includes pilot studies towards a frozen Fixed Wing ATPL.
Aerospace Engineering with Pilot Studies: BEng (Hons) ~ Undergraduate degrees ~ The University of Liverpool
Jumping into the Aviation world without a backup is a big gamble. Even though I have a Pilot Ticket already the flying is being put in second place for now.
Also just to mention lastly, some airlines run a pilot cadet scheme. Any of these programs will require the applicant to hold some nice academics on paper.
I know you are young, I was that age 6 years ago and just wanted to drink and party every weekend. I HATED studying with a passion but now I am back at college after toooo long I am loving it. Don't do what I done, get your education first then worry about being up in the clouds.
Good Luck with whatever you decide
Dave